Python Language Format literals (f-string)


Literal format strings were introduced in PEP 498 (Python3.6 and upwards), allowing you to prepend f to the beginning of a string literal to effectively apply .format to it with all variables in the current scope.

>>> foo = 'bar'
>>> f'Foo is {foo}'
'Foo is bar'

This works with more advanced format strings too, including alignment and dot notation.

>>> f'{foo:^7s}'
'  bar  '

Note: The f'' does not denote a particular type like b'' for bytes or u'' for unicode in python2. The formating is immediately applied, resulting in a normal stirng.

The format strings can also be nested:

>>> price = 478.23
>>> f"{f'${price:0.2f}':*>20s}"

The expressions in an f-string are evaluated in left-to-right order. This is detectable only if the expressions have side effects:

>>> def fn(l, incr):
...    result = l[0]
...    l[0] += incr
...    return result
>>> lst = [0]
>>> f'{fn(lst,2)} {fn(lst,3)}'
'0 2'
>>> f'{fn(lst,2)} {fn(lst,3)}'
'5 7'
>>> lst